As used herein GTL diesel process means Fischer-Tropsch synthesis in which synthesis gas is converted into liquid hydrocarbons via Fischer-Tropsch reactions, while GTL gasoline process means a process in which synthesis gas is first converted to oxygenated compounds such as methanol and/or dimethyl ether and these are subsequently converted to gasoline.
In particular, a typical GTL diesel plant consists of the following main process units: (a) air separation, (b) syngas preparation via ATR, (c) Fischer-Tropsch synthesis of a raw product of wax and liquid, (d) upgrading comprising hydrocracking and other refinery steps.
In most GTL diesel plants the main final products are diesel, kerosene, naphtha and liquid petroleum gas (LPG). In some GTL diesel plants higher value products such as lube oil are also produced. The value of the naphtha is lower than the diesel and LPG value is in most cases lower than the naphtha. In some geographic locations the LPG (mainly C3/C4 fraction) and other constituents of the light-end stream can have a very low value and limited market, hence the investment which has to be made into the upgrading of the light-end stream can be high compared with the value of the product. It is therefore known to recycle LPG to the reforming section of the plant.
WO2004/000772 concerns a process for production of a blended syngas feed. A first syngas (synthesis gas) is formed by reacting methane with oxygen, while a second syngas is formed using LPG and CO2.
AU 20073566234 (WO2009/008092) discloses the recycle of a light-end fraction. According to this citation, LPG together with naphtha can be recycled into the syngas production step. This citation discloses hydrotreating of the Fischer-Tropsch product in the upgrading unit and does not require use of oxygen in the reforming stage, i.e. the synthesis gas producing unit is not an autothermal reformer but a steam methane reformer. Further, a particular and expensive catalyst in the form of supported ruthenium is used in order to avoid deactivation of the catalyst due to deposition of carbon derived from the recycle of the light-end fraction. In particular, the number of carbon atoms in the recycle light-end fraction stream is kept at 10-35% based on the number of carbon atoms of the natural gas fed to the synthesis gas producing unit, since at above 35% carbon deposition on the catalyst will take place. Below 10% it is stated that it is not possible to improve the raw material consumption.
WO2007/101831 discloses a process for the preparation of a Fischer-Tropsch (FT) synthesis product. After pre-reforming and partial oxidation, a syngas enters an FT synthesis. The FT product is separated into heavy and light streams, and the light stream (comprising inerts, CO2 and C1-C3 hydrocarbons) is recycled. A portion of the light stream is recycled to the pre-reformer and a portion is recycled to the burner of the partial oxidation step. However, such light stream of unconverted syngas, inerts, CO2 and C1-C3 is what is normally known as FT tail gas; it is not the light-end fraction, in particular LPG, from an upgrading unit downstream FT synthesis. In addition, the light stream (tail gas) of this citation leads at least a portion of it directly to the burner of a non-catalyzed partial oxidation reactor.
WO01/60773 discloses a system and method for operating a GTL plant. In involves pre-reforming, syngas generating via e.g. autothermal reforming, FT synthesis and product upgrading steps. Light fractions from the upgrading stage, which includes hydrotreating and/or hydroisomerization, are recycled into the pre-reformer (line 162, FIG. 2). However, such light fractions are fed back precisely to the pre-reformer and after addition of steam. This can result in sulphur poisoning of the pre-reformer catalyst as well as catalyst deactivation due to carbon deposition as a result of the presence of olefinic hydrocarbons.
WO2013/033812 and US2013/0065974 disclose a process for the production of diesel via Fischer-Tropsch synthesis in which naphtha from the upgrading section is recycled as ATR feed through a pretreatment unit to which steam and hydrogen are added. Such unit may include a feed gas hydrotreater, sulphur removal and a pre-reformer and combined with natural gas feed. There is no disclosure of the recycle of light-end fractions, in particular LPG to the hydrocarbon feed, e.g. natural gas feed.
It would be desirable to provide a process that overcomes the shortcomings of the above known processes.